Hello,
I think this option is rather useless, gPhotoShow is a screen saver and runs only when the system is idle. You can prevent turning off monitor or system stanby by configuring windows power saving options, they only work when system is idle exactly when gPhotoShow is running.
Anyway gPhotoShow Pro has a plugin with this function and with the opposite one, turn off monitor (or the system) at a given time of the day.